I think the relevant iptables commands for port triggering with Bittorrent are:
#iptables -t nat -A prerouting_wan -p tcp –dport 6881:6889 -j TRIGGER –trigger-type dnat –trigger-proto tcp –trigger-match 6881 –trigger-relate 6881-6889
This tells the router to do NAT on the packets arriving on the WAN TCP ports 6881 through 6889 as allowed by the trigger on TCP port 6881.
#iptables -A forwarding_wan -p tcp –dport 6881:6889 -j ACCEPT
And this tells the router to forward packets on TCP ports 6881 through 6889 (it should forward packets that have reached here through application of the previous rule).